This invention relates to cryogenic refrigerators, or cryocoolers utilizing linear drive motors having pistons or displacers which reciprocate in cylinders. Such refrigerators include Gifford-McMahon or Stirling refrigerators and expansion engines.
In various types of cryogenic refrigerators, a working fluid such as helium is introduced into a cylinder, and the fluid is expanded at one end of a piston to cool the cylinder. For example, in Gifford-McMahon type refrigerators, high pressure working fluid may be valved into the warm end of the cylinder. Then the fluid is passed through a regenerator by movement of a displacer-type piston. The fluid which has been cooled in the regenerator is then expanded at the cold end of the displacer. The displacer movement may be controlled by either fluid pressure differentials or by a mechanical drive.
A control system for a cryogenic refrigerator is disclosed in U.S. Pat. No. 4,543,793. One or more parameters of a reciprocating-piston type refrigerator are monitored to provide an electrical feedback signal. That signal is processed to control the timing of the piston movement and or the flow of refrigeration gas into the refrigerator. Preferably, the feedback signal is an indication of the position of the piston within its cylinder and/or the temperature at the cold end of the cylinder throughout a refrigeration cycle.
Continuous position indication may be provided by a linear variable displacement transformer or by a rotary encoder or by other means. Preferably, the feedback signal is used to control valves which introduce the refrigeration gas into the cylinder or a piston drive motor. In a pneumatically driven refrigerator, the feedback signal may be used to control valves to and from both the refrigeration cylinder and the drive cylinder. By controlling the stroke, the temperature of the cold end of the refrigerator can be controlled.
A split Stirling refrigerator is disclosed in U.S. Pat. No. 4,664,685 wherein a compressor provides a nearly sinusoidal pressure variation to a refrigerant gas in communication with a cold finger. The compressor is comprised of a linear drive motor having a drive coil which drives a reciprocating armature. A detector circuit is coupled to the drive coil for sensing an electrical parameter of the coil which is a function of movement of the armature. Motor drive circuitry which applies current to the drive coil is responsive to the sensed electrical parameter in controlling movement of the piston element. The detector circuit can be connected to sense back EMF in a displacer drive motor within the cold finger.
Rotary driven compressors of cryogenic refrigerators have been known to employ controlled variations in speed to adjust the temperature at the cold end of the refrigerator.